1. Field of the Invention
The present invention relates to a biological information imaging apparatus and a biological information analyzing method. Further, the present invention also relates to a biological information imaging method.
2. Description of the Related Art
In the medical field, there have been actively studied photo-imaging apparatuses which can obtain information in a living body by causing light irradiated from a light source such as a laser onto the living body be propagated therein, and by detecting the propagation light.
One such optical imaging technique is a technique of DOT (Diffuse Optical Tomography), called diffuse optical imaging, as described in a first non-patent document described below. Diffuse optical imaging is a technique in which light is irradiated onto a living body from a light source, and the extremely feeble or weak light propagated and diffused in the living body is detected by an optical detector of high sensitivity, whereby a distribution of optical characteristic values in the living body is imaged from the detection signal.
The light, being irradiated from the light source and having passed through a relatively thick tissue of the living body, is strongly scattered, as a result of which it propagates in the tissue of the living body in a diffused manner while losing its wave nature such as straightness of light propagation. Therefore, the values of optical properties (absorption coefficient effective scattering coefficient μs′, etc.) of the tissue of the living body can be obtained by optically measuring the intensity of such diffused light at multiple points and by processing the measured values by means of a computer. Furthermore, a compositional distribution of materials that constitute the tissue of the living body can be obtained by measuring those optical property values at different wavelengths.
On the other hand, an optical imaging technique other than DOT is PAT (Photoacoustic Tomography), called photoacoustic imaging. Photoacoustic imaging is a technique that calculates a distribution of optical property in a living body with high resolution by making use of a property of ultrasonic waves having a smaller amount of scattering in the living body as compared with light.
In this method, pulsed light generated from a light source is irradiated onto the living body so as to be propagated and diffused therein, and an acoustic wave generated from the tissue of the living body that has absorbed the energy of the pulsed light is detected. By performing mathematical processing on this detection signal, it is possible to obtain distributions of optical properties in the living body, in particular, an optical energy absorption density distribution. It is said that by using this photoacoustic imaging, an optical property distribution with high resolution can be obtained as compared with the above-mentioned diffuse optical imaging.
According to a second non-patent document described below, in photoacoustic imaging, the sound pressure P of an acoustic wave obtained from an absorber in a living body due to the light or optical absorption thereof is provided by the following expression (1):P=Γ·μa·Φ  (1)
Here, Γ is the Grüneisen coefficient, which is related to an elastic property, and is obtained by dividing the product of the isobaric volume expansion coefficient β and the squared speed of sound c by the specific heat Cp; μa is the absorption coefficient of the absorber; and Φ is the amount of light irradiated onto the absorber.
Because it is known that Γ takes a substantially constant value for a given tissue, it is possible to obtain the product of μa and Φ, i.e., an optical energy absorption density distribution H, by measuring the change of the sound pressure P, which is the magnitude of the acoustic wave, by time sharing (see the mentioned second non-patent document).
[First Non-Patent Document]
A. P. Gibson, et al., “Recent Advances in Diffuse Optical Imaging”, Phys. Med. Biol. 50 (2005) R1-R43
[Second Non-Patent Document]
M. Xu and L. V. Wang, “Photoacoustic Imaging in Biomedicine”, Review of Scientific Instruments, 77, 041101 (2006)